The invention relates to a conditional access apparatus, a method of scrambling a signal, and a transmission system comprising a conditional access apparatus.
A conditional access (CA) apparatus performs a combination of scrambling and encryption to prevent unauthorized use, for example, reception of information. Scrambling is the process of rendering sound, pictures and/or data, etc. unintelligible. Encryption is the process of protecting the secret keys (control words) which have to be transmitted with the scrambled signal for the descrambler to operate, that is to say, to descramble the scrambled signal.
The primary purpose of a CA system for broadcasting is to determine which individual receiver(s)/set-top decoder(s) will be able to receive and supply particular program services, or individual programs to the viewers. After this determination, the chosen receivers will receive the information they need to operate.
Conditional access systems are known, for example, from the article xe2x80x9cFunctional model of a conditional access systemxe2x80x9d, EBU Technical Review, Winter 1995.
This article describes a functional model of a CA xe2x80x9creferencexe2x80x9d system. At the transmitting side, basically a multiplexer multiplexes picture, sound and data information, generating an MPEG-2 transport stream. This MPEG transport stream is forwarded to a scrambler. After scrambling and modulation, the signal will be sent to the receiving side. At the receiving side, the signal will be demodulated, descrambled and demultiplexed. To improve the security of the system, an improved scrambling system has been described in this article wherein the scrambler uses control words (CW) to scramble the transport stream.
These control words are generated by a control word generator. The generated control words are supplied to an encrypter for encryption of the control words under the control of a service key (for example, a smart card), and the encrypted control words are supplied as entitlement control messages (ECMs) to the multiplexer. At the receiving side, a decrypter is needed to decrypt the ECMs to obtain the CWs. Also at this side, a service key (for example, a smart card) controls the decrypter.
This prior-art conditional access system operates pretty well in a studio environment where the control word generator and encrypter are placed in a secured room. However, when the CA system has to be used on a mobile basis, the control word generator and the encrypter are no longer in secured rooms. Consequently, there will be a great risk of theft of the conditional access apparatus and/or smart card.
The invention seeks, inter alia, to provide a conditional access apparatus which overcomes the disadvantage of the prior art. To this end, a first aspect of the invention provides a conditional access apparatus as defined in claim 1. A second aspect of the invention provides a transmission system as defined in claim 3. A third aspect of the invention provides a method of scrambling as defined in claim 4. By storing the unencrypted control words and the encrypted control words, it is no longer necessary to use a control word generator and an encrypter. A processor can determine the set of control word and entitlement control message that will be used for the next predetermined time. In the case of theft of a conditional access apparatus, a new table of control words is sent to the other operating conditional access apparatuses at the transmitting side. Thereafter, the stolen conditional access apparatus can no longer be used.
The unencrypted control words and the encrypted control words can be generated centrally by an authorized person who distributes the tables obtained to the authorized users. Consequently, the conditional access apparatus will also become cheaper.
An embodiment of the conditional access apparatus according to the invention has the features of claim 2.
Beforehand, the control word(s) and the encrypted control word(s) are written into the respective storing means. During operation of the conditional access apparatus, the control means control the means for reading out the storing means. The storing means may be read out in a random order and changed as often as is deemed necessary to prevent unauthorized persons from cracking the scrambling control words.